This invention relates to a method and a device for thermal control of a roll profile in a rolling mill.
A metallic band rolling installation comprises, generally, one or several roll stands each containing at least two working rolls and associated with means for controlling the passage of a band to be rolled between the said rolls.
Usually, each roll stand comprises two supporting stanchions, spread apart and connected by crossbeams between which is mounted a set of superimposed rolls with axes parallel and placed substantially on the same clamping plane substantially perpendicular to the feeding direction of the product.
Rolling mills of different types can be made. Generally, in a rolling mill, the product to be rolled passes between two working rolls that delineate the rolling plane; the diameter of these rolls is preferably reduced in comparison to the loads to which they are exposed and these rolls rest respectively on at least two back-up rolls between which the rolling load is applied.
The so-called xe2x80x98quartoxe2x80x99-type rolling mills comprise therefore four superimposed rolls, respectively two working rolls associated, respectively, to two back-up rolls of larger diameter.
In xe2x80x98sextoxe2x80x99-type rolling mills, intermediate rolls are interposed between each working roll and the corresponding back-up roll.
Other types of rolling mill, comprising more or fewer rolls are known and used in the industry.
The rolls rest on one another along back-up lines substantially parallel and directed according to a generatrix whose profile, normally rectilinear, depends on the loads applied and on the resistance of the rolls. Generally, the clamping load is applied by screws or actuators interposed between the roll stand and the ends of the shaft of the upper back-up roll, whereas the lower back-up roll rests by its ends directly on the roll stand.
The clamping loads are applied between both ends of both back-up rolls. Since the rolled product, variable in length, does not cover the whole length of the working rolls, each roll may sag under the action of the loads applied and the thickness of the feeding space of the band between the working rolls may therefore vary, since the edges of the band could hence be thinner than the central part. These thickness defects also lead to flatness defects of the rolled band, particularly in the case of cold rolling and with thin thicknesses.
For quite a long time, one has tried to correct these thickness defects on the profile across the rolled product and various means have been used to that purpose. For example, it has been suggested to compensate for the deformation of the rolls caused by the rolling action, by cambering their surface thanks to machining to a particular profile. Its has also been suggested to perform continuous adjustable correction, by cambering the working rolls, generally small in diameter, while applying controlled deflection loads on both ends of their shaft.
More recently, it was suggested to distribute the loads on the width of the rolls by giving an adjustable profile to at least one of the back-up rolls. Such a roll comprises a deformable shell mounted rotatable around a fixed shaft on which the clamping load is applied and resting on this shaft via a set of actuators whose position and/or pressure are adjustable individually thanks to a regulation system, according to the flatness measured on the band, downstream of the rolling stand, whereas the thickness defects thus determined are compensated for by acting on the distribution of the loads over the width of the band.
These thickness defects are caused essentially by the rolls that are flattened by the load applied and the actuators suggested act on the profile of the back-up generatrix in order to modify the general aspect of the deformation, but do not allow local correction of the roll profile in cross section, at a given place.
It has also been suggested to compensate for the flattened rolls or at least the irregular flattening over the whole width, by varying the diameter obtained locally by thermal expansion.
Indeed, rolling generally produces a lot of heat due to the friction of the band rolled between the working rolls and it is therefore necessary to cool them down. In this view, at least one of the rolls, normally a working roll, is associated with a spray ramp with a heat exchanging fluid and comprising a plurality of spraying members spaced apart from another along a direction parallel to the axis of the roll and each provided with a spray nozzle with a jet of fluid directed towards one face of the roll turned to the ramp and whose flow rate is determined, for each spraying member, via a valve controlled individually by an adjustment system.
This thermal effect must be limited to the portion of the roll that covers the running band and it is the reason why the adjustment system of the flow rates determines the opening of the valve of the spraying members over a limited portion of the ramp that determines the spraying of the fluid over a cooled zone of the roll corresponding to the width of the rolled band and the closing of the valves over the remaining portions of the ramp.
Each spray nozzle is usually provided with a slot enabling to deliver a flat jet centred on a middle plane that cuts transversally the axis of the roll in order to form an oblong impact surface with a small width and extending over a portion of the height of the roll.
The cooled zone therefore consists of a series of impact surfaces substantially parallel and spaced apart from one another by a distance slightly greater than the width of each surface.
Preferably, the middle planes of the flat jets, in which are placed the greater axes of the impact surfaces are tilted with respect to the axis of the roll, so that the impact is spread, on the left and on the right, on either side of the centre of the jet, while covering a width that protrudes slightly above and beneath the centres of the adjacent jets, without interference between the impact surfaces.
Moreover, the average flow rate, per time unit, of the jet of sprayed fluid onto each impact surface can be adjusted individually by the flow rate adjustment system. It is thus possible to control accurately, per truncated zones, any variation of the profile, in cross section, of the roll over the whole length of the cooled zone, in order to modify the distribution of the loads in order to correct the flatness defects detected downstream.
Such systems have proven their efficiency, particularly for rolling thin bands and very thin bands. They have been used, initially, for rolling non ferrous metals, in particular, aluminium, owing to the low thermal inertia due to the small thickness and the malleability of the metal. However, more recently, it has been sought to apply this thermal control method to the rolling of ferrous metals.
Thanks to all these new means, the flatness quality of the rolled sheets could be improved. However, because of this improvement, residual defects, which had not been taken into account previously, have been put in evidence on the edge of the rolled bands, especially the thinnest bands.
The invention obviates this shortcoming while making improvements to the systems used until now for controlling mill rolls that enable to obtain as perfect as possible a flatness quality.
In this view, the invention uses a thermal control system of conventional type, in which at least one roll of the mill is associated with at least one spray ramp with a fluid to control, per truncated zones, the effect of the fluid jets over a cooled zone of the roll.
According to the invention, the distance between the middle axes of the impact surfaces is caused to vary in relation to the position of the said impact surfaces of the fluid jets over the length of the cooled zone so that the said zone comprises a central zone in which the middle axes of the impact surfaces are spaced by a substantially constant pitch and whereas two transition zones extend on either side of the central zone at least up to two edges of the band and in which the spacing between the middle axes of the impact surfaces is reduced with respect to the pitch of the central zone.
According to a particularly advantageous embodiment of the invention, the spraying ramp comprises a central portion with constant pitch, corresponding to the central zone of the cooled zone, in which each fluid jet is directed along an injection axis perpendicular to the axis of the roll and two lateral portions with reduced pitch, in which the orientations of the axes of the jets with respect to the axis of the roll are caused to vary, so that they may converge respectively toward two transition zones on either side of the central zone of the roll, whereby the number of convergent jet nozzles is such that, taking into account their distribution on the ramp, each lateral portion of the ramp covers a length greater than that of the corresponding transition zone of the roll.
Preferably, the middle planes of the jets directed to the central zone of the roll are tilted by a same non-zero angle with respect to the axis of the roll and the tilting angle of the middle planes of the jets directed respectively to both transition zones is increased progressively, as the corresponding impact surface is spread away from the central zone.
The invention applies therefore to a thermal control device comprising, in a known fashion, at least one spray ramp consisting of a plurality of spraying members spaced apart and fed with a heat exchanging fluid and each provided with a valve associated with an individual adjustment system of the flow rate sprayed by each spraying member.
According to the invention, as the width of the product to be rolled can vary between a minimum width and a maximum width, the spraying ramp comprises at least three series of spraying members, respectively, a central series covering a central portion of the cooled zone over a length not exceeding the minimum width of the product and in which the spraying members have fixed directions so that the axes of the corresponding impact surfaces are spaced by a constant pitch in the said central portion of the cooled zone and two lateral series extending on either side of the central series to cover, in total, a length at least equal to the maximum width of the product and in which the spraying members are mounted pivoting on the ramp, whereas each lateral series is associated with a means for adjusting the orientation of at least one group of pivoting spraying members, in order to reduce the spaces between the axes of the impact surfaces in a transition zone at each end of the cooled zone of the roll.
According to a preferred embodiment, as the band to be rolled is centred in a longitudinal plane of symmetry of the mill, each lateral series of spraying members comprises, considered from the inner side to the outer side, a first section in which the axes of the jets are at right angle to the axis of the roll and that covers a first lateral portion of the cooled zone of the roll over a length such that the total length of the central portion of the cooled zone, increased by the said first lateral portions is smaller than the width of the band, and a second section in which the axes of the jets are tilted inwardly with respect to the axis of the roll and that covers a second lateral portion of the cooled zone over a length such that the total length of the said cooled zone is at least equal to the width of the band, whereas each second section of a lateral series covers, at one end of the cooled zone, a transition zone corresponding to an edge of the band and in which the middle axes of the impact surfaces are spaced by a distance smaller than the spacing pitch of the said surfaces, respectively in the central portion and the first lateral portions of the cooled zone.
According to a particularly advantageous embodiment, the adjustment means for orienting the jets comprise two means for controlling the pivoting of one group of spraying members, respectively on each lateral series, whereas each control means is moveable along the ramp and associated with a means for adjusting its position in relation to the width of the band and with a means for selective engagement of the said control means with a group of spraying members making up a second section of each lateral series to cover a transition zone, at each end of the cooled zone.
Usually, each spraying member comprises a tubular body exhibiting an outlet end provided with a jet formation nozzle and an inlet end connected to the duct via a connection piece delineating a junction channel between the inner side of the duct and the inlet end of the tubular body, on which is placed a valve connected individually to the adjustment system.
According to the invention, in each lateral series of the ramp, each spraying member comprises a tubular body mounted pivoting on the connection piece around at least one axis orthogonal to the axis of the roll.
To adjust the orientations of the jets, each lateral series of spraying members is associated with a means for selective control of the pivoting of a group of spraying members comprising a cursor provided with fingers spaced apart and mounted to slide on a support, along an axis parallel to the feeding duct of the ramp, a means to control the sliding motion of the cursor on its support for adjusting the position of the cursor along the ramp and a means to control the rotation of the cursor around its axis into two opposite directions, respectively, an engagement and a disengagement directions of the cursor fingers between the tubular bodies of a group of spraying members of the ramp.
According to a preferred embodiment, the fingers of the cursor are spaced apart by a constant distance that is slightly smaller than the distance between the axes of the tubular bodies of two neighbouring spraying members, whereby the said fingers of the cursor rest one after the other on the said tubular bodies as the cursor is sliding, in order to determine progressive variation of the tilting angles of the jets with respect to the axis of the roll.